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...what do you call it when the car carries on running for a little bit when you turn it off?

I saw 'overrun' used in another thread to do with rumbling prop at speed.

And I'm experiencing some of the second question whenever I switch off after really gunning the car.

Cheers,
Ben

Ben, you are experiencing 'running on' where the engine continues for a short(ish) period to run. This can be caused by a few different things but is usually connected with the engine being hot with sources of heat in the engine providing sufficient heat to ignite the fuel/air mixture. It is also more prominent in cars with high compression ratios. Remember diesels son't have spark plugs just rely on sufficient compression of the fuel/air mixture. Our cars can suffer from run on if there is sufficient heat and maybe timing inaccuracy to keep the engine firing for a few revolutions. There are threads where this is discussed in depth.

The thread where you read 'overrun' refers to a different scenario. The overrun in this context means the point when you take your foot of the gas and the car is coasting (i.e. not accelerating and the drive-train is not under stress).

I hope this makes sense.

Cheers, Neil

Excellent, thanks Neil. I'll do a search for some of the threads you mention.

Should explain 'Expensive'.

Gave mine a decoke...had the valves re-cut and the seats replaced...skimmed. About &163;350!

Doesn't run on at all now and I have been using 95 octane.

Pete, I think that running on can often sound like a diesel as mine used to do the same a little before I sorted it out. I also think that it is possible for the engine to turn in the opposite direction as the combustion of the air/fuel mixture is not being controlled by the distributor and the rest of the ignition system. The combustion of the gases is happening as there are sufficient sources of ignition to ignite the mixture and the point that these occur will determine which way the engine will rotate. The engine if still rotating will draw fuel/air into the engine and compress the mixture and .................... Someone may have some greater pearls of wisdom on this but I can imagine that this could occur.

Look through some of the threads on this subject as I am sure there must be some explanations of what the causes are and ways of overcoming the problem.

Regards, Neil

When an engine 'runs on' after you have turned off the ignition it will indeed sound like a diesel, this is because the spark plugs are no longer providing ignition but the compression alone is igniting the fuel - this is exactly how a diesel engine works.

Teccy bit... all hydrocarbon fuels have a 'knock' rating this is a measure of the amount you can compress the fuel to make it spontaneously combust. For petrol this is very high and so a spark plug is needed to provide the ignition. However, in diesel it is quite low, when you first turn the ignition a glow plug provides ignition but as the engine gets hot the compression alone will be able to do this.

Consider then that if a spark plug provides the ignition then the 'bang' begins from a single point and expands from there in a wave across the piston - thus pushing it down - this is why we need to advance the timing so the wave meets to piston at exactly the right time as the piston is ready to begin it's decent. If the pressure wave arrives too early (advanced) then it hits the piston on it's way up and causes pinking. This puts extreme stresses on the piston and can destroy it (see my article in October Courier). In a diesel though, the compression alone provides ignition and there is no pressure wave as the fuel ignites spontaneously. This makes a very different sound and that's why diesels sound so different - and isn't that the sound you hear when your engine 'runs on'?

The reason your engine is running on can be two fold... there is a 'hot spot' (eg rough edge or carbon) that is acting like a glow plug and will ignite the fuel, or the chamber temerature is high enough to allow your fuel to reach it's knock rating. The octane rating of the fuel you use is a reference to it's knock rating, the lower the number the lower the knock rating. This is why low octane fuels can cause this problem and they can be solved by using a higher rating.

This is the same reason why racers use 97/98 (or 100) octane fuel as we want more power by increasing the compression ratio and so need use fuels with higher knock ratings.

The engine will NOT however run backwards, the valves will not allow any fuel/air into the chambers and the engine cannot run on exhaust gasses. Which ever reason for your engine overruning the ignition source will not be so far advanced to cause the engine to reverse.

Thanks Nig for the explanation. I hadn't thought of the valves and stuff but just in my little head could conceive that it was possible.

Neil

...what do you call it when the car carries on running for a little bit when you turn it off?

Expensive!

My engines runs on a bit after i've switched it off only if I have been using 95 Octane unleaded .. it is totally fine on Super

I have been putting 95 in. I was using 97 but thoght I'd try 95 + valvemaster octane boost lead replace jobby.
That could well be why its only happened in last week or so.

Ben I think many people have found the same using lower octane unleaded. I think the GT6 was one of the cars that suffers this more than most as the engine was intended for use with 5* fuel. Of course it all depends on how your engine has been modified and what the timing is set to. Others with greater knwoledge than I will probably be able to confirm that if your ignition timing is retarded slightly then this will minimise the effect but may also reduce performance. Also advancing the timing may bring on pinking (pre-ignition) so it is diffcult to get the balance right. I would set the timing to get a small amount of pinking when putting the engine under load in top gear at 30ish. From what I have read this is acceptable and that should prevent getting running on. If you still get running on then you will have to use the higher octane fuel. I tend to use BP ultimate (etc) anyway. I have never been able to tell whether the car benefits from it or whether I have been wasting more pennies. I did feel that the car gives a couple of extra MPG with so the costs balance out but it is very hard to tell.

All the best, Neil

My car used to run on a bit, but it looked like it was spinning the wrong way (and sounded like a deisel). Is this normal/ Possible?

Cheers, Pete

Hallo... I just thought I'd confirm your comments about octane level. Last night I thought I'd try and put some 97 in at the local Tescos, and when I pulled up found that they have JUST started to do 99ron... nice! So yea, I filled the remaining half a tank with that and at the 2 'switch-offs' I've done since then, absolutely no run-on. And it was not as expensive as I thought it would be, about 94p I think.

I dont know if this is likely, but i could swear the car is happer idling too. Normally at 1k rpm its rather ropey sounding, but while i was packing some boxes the car was idling happily at a constant tone.

[is it possible to have a placebo effect with a car?

]

Ben there definitely is the placebo effect when it comes to our cars as they are human just like us and can be fooled and conned just the same!

Cheers, Neil

Yes Pete, it can run backwards and it sounds different from "normal" dieseling - more buzzy. If it's in gear with handbrake on and you let the clutch up to try and stop it running it will try and move a bit but not the way you expect it to!
Van

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Terminology: Whats overrun? and...

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